The present invention relates to a polyester film for decorative plates or decorative sheets. The term "decorative plates" used in the present invention usually means building materials (facing materials) which comprise at least a film layer and a picture-printed layer laminated successively on the surface of a base of various kinds of commercial products such as, typically, furniture, building materials and home appliances, and which building materials are handled as an independent commercial product. The "decorative sheets" mean the decorative materials (facing materials) which have a picture printed surface layer and which are laminated to the surfaces of the bases. Therefore, the film layer and the picture-printed layer formed on the base surfaces of the decorative plates are an element constituting the decorative sheets.
Vinyl chloride resin sheets are the most popularly used as the sheet interposed between the base and the picture-printed layer of the decorative plates, and as the sheet (film) used for the decorative sheets. Practical use of such vinyl chloride resin sheets, however, involve some serious problems. For instance, the plasticizer contained in the sheets may migrate into the adhesive layer at the interface of lamination to cause improper or imperfect adhesion at the interface. Also, such vinyl chloride resin sheets are liable to thermal expansion and contraction because of their poor thermal dimensional stability and may be wrinkled when exposed to a cold or hot environment. Further, these vinyl chloride resin sheets generate chlorine when incinerated and the waste thereof is considered responsible for acid rain and generation of dioxin. Under these circumstances, request is rising for the development of the decorative plates and decorative sheets using no such problematic vinyl chloride resin sheets from the standpoint, in particular, of environmental protection.
Since the decorative plates or decorative sheets are used as facing material, usually a picture with a sophisticated or aesthetic design effect is applied on their surface. Therefore, proper adjustment of tonality is essential for producing a fine contrast of the picture or shading. A large variety of materials such as, typically, particle board and steel plates are used for the base of the decorative plates or the base to which the decorative sheets are to be laminated, and the tonality of these bases is also diversified even if they are made of a same material. Therefore, if the opacifying or hiding power of the sheet material used for the decorative sheets is low, the tonality or color tone of the base affects the tonality of the decorative sheet surface to spoil its original ornamental effect.
Therefore, a decorative sheet (film) having proper opacifying properties is desired for allowing wider application of such sheet to the bases of various tints.
Ligneous materials are preferred for furniture, building materials, home appliances and or the like, for which the decorative sheets are used as facing material. Therefore, it is desired that a sheet material of a tinge capable of affording a ligneous impression be used for the decorative sheets.
Further, since the decorative plates or sheets are used as facing material, there is high probability that such plates and sheets would contact or touch various other objects in use. For instance, in case where the decorative plates or sheets are used as facing material of a door in construction of a house, they are subjected to frequent occasions of contact with, for example, the tools, machine parts and such carried in by the house builder, or with the corner of a furnishing when brought into the room. Even in the everyday life, there are many occasions of such contact, for example, bumping of the corner of the intake block of a vacuum cleaner against the facing material. Such contact with other objects often causes fouling or scratch to the decorative sheet. Here, the term "fouling" is used to refer to a mark which has been transferred from the contacted object to the decorative sheet surface and distinguished from the original picture of the decorative sheet. The "scratch" means damage creating unevenness or dent on the sheet surface which does not harmonize with the sheet picture. It is distinguished from the picture since light reflection from the uneven portion is different from that from the surrounding part of the picture. Both fouling and scratch deteriorate the design effect of the decorative plates and sheets. Especially scratch causes a serious problem in which it damages the decorative sheet surface and which problem can not be eliminated unless the sheet is changed. Thus, a decorative plate or sheet which is resistant to damage by contact with other objects has been desired.
In a picture printer, there are provided, separately from the rolls for adjusting the film tension, the rolls for preventing the printed picture on the film from contacting the drying oven. So, if the color of the film surface is transferred to the roll surface, the hue of the film surface is altered, making it difficult to control tonality. Productivity is also hampered as the rolls need to be cleaned. Thus, in the industry for a decorative plate or sheet, there is a strong request which, in its production using a picture printer, allows easy control of tonality of the film surface and which is also capable of preventing color transfer to the roll surface in the picture printer, and there is no problem in which the productivity is impaired by the external factors.
Biaxially oriented polyester films represented by polyethylene terephthalate film are used in many fields for their excellent properties. However, no proposal has ever been made on use of a biaxially oriented polyester film as an intermediate between the base and the picture-printed layer, which polyester film is capable of preventing color transfer to the roll surface in the picture printing operation, although there have been a proposal on use of a polyester film as a facing member of decorative plates in Japanese Patent Application Laid-Open (KOKAI) No. 7-17005 and a proposal on application of a polyester film for a colored opaque resin-made decorative sheet in Japanese Patent Application Laid-Open (KOKAI) No. 5-116251.
As a result of the present inventors earnest studies to solve the above problems, it has been found that in the production of decorative sheets comprising at least a polyester film layer having a specified migration value and a picture-printed layer, and the decorative plates comprising the said sheets laminated to a base, it is possible to prevent transfer of the color of the film surface to the roll surface in the picture printer; and that the decorative sheets comprising a polyester film layer having a specific transmission density and chromaticity, a picture-printed layer and a surface hardened layer having a specific Rockwell hardness and the decorative plates comprising such sheets laminated to a base are capable of presenting the high design quality and also highly resistant to damage.
The present invention has been attained on the basis of the above finding.